In a radar transceiver, for example, outgoing microwaves are conventionally frequency-modulated according to a periodic law which must also be used in concurrent modulation of a heterodyning signal used for demodulating incoming waves reflected by an external target. Thus, a constant frequency difference .DELTA.f must be maintained at all times between these two variable oscillations. The outgoing high-frequency waves, radiated by a directive antenna, are sometimes generated by a coaxial magnetron which is tunable with the aid of a reciprocating piston. An eccentric shaft portion of a servomotor is linked with the piston by a crank drive to translate a rotation of that shaft into a linear motion of the piston. With such a crank drive, however, the angular displacement of the shaft from a reference position is not strictly proportional to the piston stroke. Thus, the piston does not lie exactly midway between its two dead-center positions when the shaft has been turned through 90.degree. from such a dead-center position.
Since the operating frequency of the coaxial magnetron is a function of the effective volume of its resonant cavity which changes with the piston stroke, it is convenient to use an angular resolver which senses the current rotary position of the motor shaft and emits a feedback signal serving as an indication of the instantaneous operating frequency. A comparator, receiving this feedback signal along with a pilot signal from a tuning-control circuit, sets the servomotor in a position corresponding to the desired operating frequency by generating an error signal corresponding to the difference between its two input signals. The same feedback signal can also be used for the concurrent tuning of a voltage-controlled oscillator (VCO) generating the heterodyning signal. Proper correlation of the operating frequency of the magnetron and the VCO, however, requires a linearization of the relationship between the piston stroke and the shaft rotation. Various mechanical linearizers are already known for this purpose. There have also been proposals for linearly operating crank drives; see commonly owned U.S. applications Ser. No. 8,091, filed Jan. 31, 1979 by one of us (Guido Busacca) jointly with three others, and Ser. No. 53,275, filed June 29, 1979 by two of us (Guido Busacca and Vincenzo Meli) jointly with one other now U.S. Pat. Nos. 4,234,855 and 4,247,828, respectively.